Plasmopara obducens – a New Threat to the Production of Impatiens walleriana in Serbia

Abstract

During 2010, Impatiens walleriana plants with symptoms of downy mildew were collected in a greenhouse in the vicinity of Mionica, Kolubara District. Disease incidence was extremely high, approaching 100%, and wilting and collapse of affected plants was very rapid, resulting in losses of more than 90%. White downy growth produced on the lower leaf surface consisted of hyaline, thin–walled sporangiophores with monopodial branching and numerous, ovoid and hyaline sporangia. Apical branchlets of sporangiophores were at right angles to the main axis, with no apical thickening. Pathogenicity tests included inoculation of young I. walleriana plants by spraying with a sporangial suspension, and downy mildew symptoms were observed after 13 to 15 days. The absence of well–defined spots on the infected impatiens leaves and straight sporangiophores indicated that the pathogen was P. obducens, which was further supported by molecular identification, the 5’–end of the nuclear DNA coding for the large ribosomal subunit (LSU rDNA) was amplified by PCR, using primers NL1 and NL4. A representative isolate, 28–10, was sequenced and phylogenetic analysis showed its grouping with other P. obducens isolates of different origin.Considering that impatiens downy mildew in Serbia is proved to be caused by P. obducens it is necessary to employ adequate phytosanitary measures to prevent further spread of the pathogen.

Keywords

• Impatiens walleriana
• Downy mildew
• Morphology
• Molecular identification
• Phylogenetic analysis